Redox reactions definition

The generalization was based on the introduction of the concept of donor-acceptor pairs into the theory of acids and bases this is a fundamental concept in the general interpretation of chemical reactivity. In the same way as a redox reaction depends on the exchange of electrons between the two species forming the redox system, reactions in an acid-base system also depend on the exchange of a chemically simple species—hydrogen cations, i.e. protons. Such a reaction is thus termed proto lytic. This approach leads to the following definitions ... 

The phrase OIL RIG can be used to help remember this definition. Because reduction and oxidation occur in the same reaction, such processes are called REDOX reactions. 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

As depicted in Figure 2.3, electrons are transferred from the oxidation step to the reduction step of the redox reaction. The number of electrons exchanged is the fundamental basis for establishing the stoichiometry of the redox process. This fact is crucial when establishing a mass balance, as will be done by modeling sewer processes (cf. Chapters 5 and 6). The OX value is, by definition, a key element in determination of this number. 

Definition of symbols AEp = peak potential difference, Epa = peak potential at cathodic peak current, Epc = peak potential at anodic peak current, tpa = anodic peak current, ipc = cathodic peak current, s = scan rate, t = time after peak (the Cottrell region), n = number of electrons involved in redox reaction. Rate parameters (acn ) and heterogeneous rate constant can be found from irreversible wave. 

Depending on the context, oxidation can either refer to (a) a reaction in which oxygen combines chemically with another substance or (b) any reaction in which electrons are transferred. For the latter definition, oxidation and reduction always occur simultaneously (redox reactions), and the substance that gains electrons is termed the oxidizing agent. Electrons might also be displaced within a molecule without being completely transferred away from it. 

Combination reactions are reactions in which two or more reactants (elements or compounds) combine to form one product. Although these reactions may be of a number of different types, some types are definitely redox reactions. These include reactions of metals... 

Q V21 Write three different definitions for a redox reaction. 

For the electron transfer of hydrated redox particles (the outer-sphere electron transfer), the electrode acts merely as a source or sink of electrons transferring across the compact double layer so that the nature of the electrode hardly affects the reaction kinetics this lack of influence by the electrode has been observed for the ferric-ferrous redox reaction. On the other hand, the electron transfer of adsorbed redox particles (the inner-sphere electron transfer) is affected by the state of adsorption so that the nature of the electrode exerts a definite influence on the reaction kinetics, as has been observed with the hydrogen electrode reaction where the reaction rate depends on the property of electrode. 

Biologically mediated redox reactions tend to occur as a series of sequential subreactions, each of which is catalyzed by a specific enzyme and is potentially reversible. But despite favorable thermodynamics, kinetic constraints can slow down or prevent attainment of equilibrium. Since the subreactions generally proceed at unequal rates, the net effect is to make the overall redox reaction function as a imidirectional process that does not reach equilibrium. Since no net energy is produced imder conditions of equilibrium, organisms at equilibrium are by definition dead. Thus, redox disequilibrium is an opportunity to obtain energy as a reaction proceeds toward, but ideally for the sake of the organism does not reach, equilibrium. 

In classical definitions a distinction is made between redox reactions and acid-base-t e reactions in the widest sense. However, no clearcut line can be drawn (7). Although it is well known that any change in coordination usually causes a chcinge in redox properties, the detailed relationships between coordination and redox properties have not been precisely formulated until recently (2, 3). 

The reaction of magnesium and oxygen is an example of an oxidation reaction. The combination of an element with oxygen was the traditional way to define an oxidation reaction. This definition of oxidation has been broadened by chemists to include reactions that do not involve oxygen. Our modern definition for oxidation is that oxidation takes place when a substance loses electrons. Anytime oxidation takes place and a substance loses one or more electrons, another substance must gain the electron(s). When a substance gains one or more electrons, the process is known as reduction. Reactions that involve the transfer of one or more electrons always involve both oxidation and reduction. These reactions are known as oxidation-reduction or redox reactions. 

Today, many reactions in aqueous solutions can be described as oxidation-reduction reactions (redox reactions). Oxidation is the process in which the oxidation number of atoms increases. Reduction is the process in which the oxidation number of atoms is decreased or made more negative. In another definition, oxidation is the loss of electrons by an atom, and reduction is the gain of electrons. Let us look at the following reaction ... 

One of these, electron transfer, actually occurs in the ideal definitional sense. It applies to the few overworked redox reactions where there is no adsorbed intermediate. The ion in a cathodic transfer is located in the interfacial region and receives an electron (ferric becomes ferrous) without the nucleus of the ion moving. Later (perhaps as much as 10-9 s later), a rearrangement of the hydration sheath completes itself because that for the newly produced ferrous ion in equilibrium differs (in equilibrium) substantially from that for the ferric. Now (even in the electron transfer case) the ion moves, but the definition remains intact because it moves after electron transfer. The amounts of such small movements (changes in the ion-solvent distance for Fe2+ and Fe3+ ions in equilibrium) are now known from EXAFS measurements. 

Almost all of the reactions that the practicing inotganic chemist observes in the laboratory take place in solution. Although water is the best-known solvent, it is not the only one of importance to the chemist. The organic chemist often uses nonpolar solvents sud) as carbon tetrachloride and benzene to dissolve nonpolar compounds. These are also of interest to Ihe inoiganic chemist and, in addition, polar solvents such as liquid ammonia, sulfuric acid, glacial acetic acid, sulfur dioxide, and various nonmctal halides have been studied extensively. The study of solution chemistry is intimately connected with acid-base theory, and the separation of this material into a separate chapter is merely a matter of convenience. For example, nonaqueous solvents are often interpreted in terms of the solvent system concept, the formation of solvates involve acid-base interactions, and even redox reactions may be included within the (Jsanovich definition of acid-base reactions. 

Values of E° by definition refer to conditions under which all species are in their standard states at 298 K. For non-standard conditions the electrode potential, E, of a redox reaction is given by the familiar Nernst expression (equation 24), where... 

This is a very unsatisfactory definition because many oxidation-reduction or redox reactions do not involve changes in hydrogen or oxygen content, as the following example illustrates ... 

Redox reactions are better defined in terms of the concept of electron transfer. Thus an atom is said to be oxidized if, as the result of a reaction, it experiences a net loss of electrons and is reduced if it experiences a net gain of electrons. This simple definition can be used to identify oxidation or reduction processes at carbon in terms of a scale of oxidation states for carbon based on the electronegativities of the atoms attached to carbon. The idea is to find out whether in a given reaction carbon becomes more, or less, electron-rich. We will use the following somewhat arbitrary rules ... 

Table III shows the abundance of various elements in the earth s crust and the oxidation states they frequently occupy. The table indicates that of the 14 most abundant elements, only six participate in redox reactions in the surface layers of the earth. [PH3 seems to be extremely rare (42) and will not be discussed.] Because by definition free oxygen as 02 is absent in the anoxic zone, it is evident that oxides of Fe(III) are the most important oxidizers in anoxic environment and that S042 and higher oxides of manganese are of importance only locally. Reducing compounds of importance are organic matter and sulfides, the latter frequently from volcanic emanations. Hydrogen is commonly combined with other elements, as in H20, CH4, and NH3 but may locally occur free as H2. Since iron is the most widespread element that can serve as an oxidizer in the anoxic environment the distribution of the valence states of iron in various rocks is of interest (see Table IV). Sandstones frequently have a high Fe203/Fe0 ratio, but shales and clays may also be highly oxidized as shown in Tables IV and V. Since approximately 75% of the earth s surface is covered with sediments and since the sediments...

Sufficient DO data were not obtained from basalt-synthetic Grande Ronde groundwater experiments to allow determination of a definitive rate law. A first order kinetic model with respect to DO concentration was assumed. Rate control by diffusion kinetics and by surface-reaction mechanisms result in solution composition cnanges with different surface area and time dependencies (32,39). Therefore, by varying reactant surface area, determination of the proper functional form of the integrated rate equation for basalt-water redox reactions is possible. 

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